Dock levelers are utilized in the transfer of goods between a building and a trailer by bridging the gap from the building floor to the trailer bed. Dock levelers include a frame or support structure for mounting the leveler in a pit of a loading dock. The rear end of a conventional dock leveler is hinged to the building floor. The opposite end has an extendable lip plate that pivots out and onto the trailer bed. Levelers are adapted to move from a generally horizontal position where the upper surface of the ramp is flush with the surface of the building floor to a second generally inclined position to provide a ramp between the bed of the truck and the dock floor.
Conventional methods of lifting the dock leveler use springs, hydraulics, and more recently low pressure air. All methods are utilized to raise the ramp assembly from a generally horizontal dock level position to an upwardly inclined position to allow the extension of a hinged lip assembly above the bed of a truck to allow lowering onto the truck. In the case of the air powered dock leveler the operator typically pushes and holds a control button until the ramp is fully raised. The lip can either extend before the ramp is fully raised or is extended as the ramp descends. Various mechanisms are used to extend the lip. By releasing the control button, the operator allows the ramp to lower under gravity. As the ramp lowers, the lip remains extended until unlatched by the bed of the truck or is retracted in a controlled manner as it enters the loading zone height. The truck supports one end of the leveler. Goods are then transferred between the building and the truck utilizing the ramp. After the trailer is loaded or unloaded, the operator pushes the control button to raise the ramp until the lip retracts to a pendent position. The operator then releases the button to allow the dock leveler to drop down to its stored position, which is level with the building floor.
Hydraulic dock levelers can be efficient, but are generally more costly to purchase and service. They are also prone to fluid leaks. Examples of hydraulically operated dock levelers are shown in U.S. Pat. Nos. 4,365,374 and 4,641,388. Spring powered dock levelers are often less costly, but have many high wear components and are more difficult to operate. Because the spring continuously biases the ramp toward a raised position, a hold down device is needed to hold the ramp at a desired stored or truck engaging position. Each time the operator wants to raise the ramp, he or she has to bend over to reach a hold down release that is typically located slightly below the floor. Once the ramp rises, the operator has to walk on the inclined ramp to overcome the force of the spring and lower the ramp down onto the truck or into its stored position.
Low pressure air powered dock levelers are generally less costly than hydraulic units and easier to operate than spring powered levelers. Examples of this type of leveler are shown in U.S. Pat. Nos. 5,042,103, 5,446,938, 5,471,693 and 5,500,968 and U.S. Publication No. 2003/0204921. These levelers include an inflatable bag or column. One problem with these designs is that they require bags with multiple sections and complex methods of construction that increases material, manufacturing and assembly cost, as well as increase friction between parts and the potential for rupture or failure. Another problem is that other components rub against the bag during operation. This rubbing or friction causes wear that leads to leaks in the bag or its rupture. This friction results from the movement of the bag to accommodate the arced movement of the ramp relative to the pit floor or support pan. Another problem with these designs is that they rely on the pit floor to form the bottom support for the bag, which requires complex installation and shipping methods, particularly to protect the bag from damage.
The present invention is intended to solve these and other problems.